In order to obtain castings with desired metallurgical properties, it is at times necessary to treat molten metal with an additive prior to its introduction to the casting cavity of a foundry mold. Herein, the term casting cavity means the cavity portion of a foundry mold in which poured metal solidifies to form useful castings along with the associated runner system. The term excludes the pouring basin and downsprue mold portions unless otherwise noted.
A widely used practice involving the introduction of an additive to molten iron is that used to make nodular or compacted graphite iron from molten iron that would otherwise solidify as grey iron. In grey iron, the graphite precipitates in flake form. In nodular iron, however, the free carbon precipitates in the form of microscopic spheroids or nodules of graphite. Compacted graphite (c.g.) iron has a graphite structure between grey and nodular irons. At least a portion of the free carbon is present in the form of elongated or lamillar type structures.
Nodular and c.g. irons are generally made by treating molten grey iron with an additive containing magnesium in alloyed or elemental form. Within limits well defined in the art, it has been found that a certain amount of retained magnesium (approximately 0.35 weight percent) will produce nodular iron while lesser amounts yield c.g. iron or iron with a mixture of compacted and nodular graphite structures.
Before this invention, molten iron has been treated with magnesium containing additives either in the pouring ladle or the foundry mold. The ladle treatment method is wasteful of expensive additive materials and has inherent processing problems. As a consequence, the inmold inoculation method has become more prevalent. The molds used in this method have at least one chamber for retaining nodularizing additive. The chamber is located downstream of the pouring basin and sprue to prevent the violent reaction which takes place when molten iron contacts magnesium or magnesium alloy in the presence of oxygen. A disadvantage of in-the-mold inoculation has been that the treatment chamber occupies mold space that could otherwise be used for good castings. Extra metal must be poured to assure uniform nodularizing treatment, but metal that solidifies in the treatment chamber is scrap. A further disadvantage to the system is that the chambers are not visible once the cope mold is set on the drag. Once the cope is set, it is impossible to visually determine whether additive has been introduced to a particular mold before or after the iron is poured. Failure to inoculate a mold will produce a grey rather than a nodular iron casting.
A number of solutions have been proposed to circumvent the need for a treatment chamber in the mold. They all involve the use of a separate secondary foundry mold consisting of a pouring basin, downsprue, treatment chamber and outlet. The secondary mold is positioned above the primary mold. The iron is poured directly into the secondary mold and is treated before it reaches the primary mold. See, for example, U.S. Pat. No. 3,819,365 to McCaulay and Dunks.
The use of a secondary treatment mold is undesirable for a number of reasons. Obviously, the manufacture of separate treatment molds is costly. From a processing standpoint, the iron must be poured at an undesirably high temperature to avoid premature solidification in the primary mold. Additional equipment is required to support the secondary mold above the primary mold.